How to Cool the Planet: Geoengineering and the Audacious Quest to Fix Earth's Climate

ONE The Prophet I GREW UP in California, where human ingenuity is a force of nature. Computers, the Internet, Hollywood, blue jeans, the Beach Boys - they are all inventions of my home state. The economic and cultural power of these things is obvious. What's less obvious is how they transformed the place that gave birth to them. Until the early 1970s, my hometown of Silicon Valley was mostly orchards and Victorian ranch houses, with rows of cherry and apricot trees that marked the coming of spring with delicate white and pink blossoms. During the PC revolution, I watched those orchards fall to make room for glassy high-tech office buildings. The hillside where I saw the footprint of a mountain lion in the 1970s is now cluttered with houses. Silicon Valley is still a beautiful place, but the blossoms are mostly gone, the sky is hazy, and the beaches are crowded. This is happening everywhere, of course - it's the story of modern life. And there are many upsides to this transformation, including the fact that the ideas and technologies born in California have been a great boon to humanity. But you have to be pretty obtuse to grow up in a place like Silicon Valley and not be aware that progress sometimes comes at a price.
 I left the Valley in my midtwenties and moved to New York City to begin a career as a journalist. My connection to the Valley served me well. I spent the next decade or so writing about the business and culture of my hometown for publications such as Rolling Stone and the New York Times Magazine. But my perspective changed after I became the father of three kids. The future of digital culture was suddenly much less interesting to me than the survival of the human race. I spent a lot of time with climate scientists while I was reporting my previous book, which was about the coal industry. It was a sobering experience. I think of myself as an optimistic person, but the deeper you probe into the climate crisis, the darker the story gets. It's hard not to read it as a parable about the dangers of living in a high-tech society. (No matter how hard they tried, a world of hunter-gatherers could not cook the planet.) And it's harder still not to wonder whether the smartest, most technologically sophisticated creatures that ever existed on earth will figure out a solution for this looming catastrophe. My friends in Silicon Valley are sure we can. They believe we are one big idea - Thin film solar! Cellulosic ethanol! High-altitude wind power! - away from solving this crisis. I used to think that, too.
 In early 2006, a friend emailed me an essay by Paul Crutzen that was about to be published in an academic journal. Crutzen is a Dutch atmospheric chemist who won the Nobel Prize for his pioneering research on the ozone hole in the atmosphere. In his note, my friend - a successful entrepreneur in the solar power industry - wrote: “Read this. We are in deep trouble. We're going to geoengineer the damn planet now!”
 I may have heard the word “geoengineer” once or twice before, but I knew next to nothing about it, other than the fact that it generally referred to people with outlandish ideas about how to counteract global warming. I had a vague memory of reading an article about a handful of scientists - I imagined them toiling in a lab buried deep in a mountain somewhere in New Mexico - who wanted to launch mirrors into space or dump iron into the ocean in a desperate attempt to cool the earth. The title of Crutzen's essay certainly amused me: “Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?” The phrase “albedo enhancement” sounded like a procedure a surgeon might perform on a lonely middle-aged man.
 When I started to read, however, I was captivated. The basic facts were familiar: carbon dioxide (CO2) levels in the earth's atmosphere are rising to concentrations not seen in twenty million years, with no end in sight. Meanwhile, the earth's climate is warming even faster than scientists had predicted just a few years ago. What was new in Crutzen's paper - new to me, anyway - was the view that some of this accelerated warming was driven not only by high levels of CO2 but also by the progress we have made in the fight against smog and other traditional pollutants. The tiny particles that cause some kinds of air pollution act like mirrors in the sky, reflecting sunlight away from the earth, which cools the planet. As we eliminate pollution, the particles vanish, letting us all breathe easier - but also letting more sunlight in, which heats up the earth ever faster. As Crutzen pointed out, by trying to save kids from asthma, we were inadvertently making the climate crisis worse.
 What to do? Clean air is obviously a good thing: air pollution kills people. The simplest solution would be to cut greenhouse gas emissions. If anyone should have been confident that we could take bold action to address this problem, it should have been Crutzen. After all, he was in part responsible for the fact that the leading nations of the world had come together in the late 1980s to confront another global threat, the ozone hole. In that case, once the risk of ozone damage was clear, action was swift: an international treaty, the Montreal Protocol, was negotiated and signed in 1987, banning ozone-depleting substances. It was an inspiring example of political leaders from around the world coming together to confront a grave threat in a rational and decisive way. But when it came to dealing with greenhouse gases, Crutzen was not so sanguine that a political solution could be found. He understood that the problem of reducing greenhouse gases is far deeper and more complex than eliminating chlorofluorocarbons from refrigerators and air conditioners, in part because greenhouse gas emissions are, in some ways, a proxy for economic health and prosperity. In fact, Crutzen called the notion that industrialized nations would join together and significantly reduce emissions “a pious wish.”
 Instead, Crutzen offered a radical proposal: rather than focusing entirely on cutting greenhouse gas emissions, maybe it was time to think about addressing the potentially catastrophic consequences of global warming in a different way. If the problem is too much heat, an obvious solution would be to find a way to reduce that heat. One method to do that would be to increase the earth's reflectivity in ways that would not cause asthma attacks and kill people. As Crutzen knew as well as anyone, about 30 percent of the energy from sunlight that hits the earth is immediately reflected back into space, while the other 70 percent is trapped here by CO2 and other greenhouse gases, warming the planet. If we could reflect just 1 or 2 percent more sunlight away from the earth's surface, it would be like popping up an umbrella on the beach on a hot summer day. Crutzen called it albedo enhancement (“albedo” is just another word for reflectivity).
 There are lots of ideas about how one might deflect sunlight away from the planet, from launching mirrors into space to painting roofs white. But as Crutzen pointed out in his paper, the simplest way to do it might be to add a relatively small number of sulfate particles - you can think of them as dust - to the upper atmosphere. The dust would remain in the stratosphere for only a year or so before raining out - so any serious geoengineering scheme would require continuous injection. But unlike pollution in the lower atmosphere, which is where the nasty stuff we breathe resides, pumping a modest amount of particles into the upper atmosphere would pose little danger to human health. The effect they might have on the chemistry of the stratosphere, especially the ozone layer that protects the earth from the sun's ultraviolet light, was, Crutzen admitted, unclear. However, his preliminary calculations suggested that the risks were low.
 Would it work? On a scientific level, there is nothing complicated about it. Light colors reflect sunlight; dark colors absorb it. That's why asphalt is hot on your bare feet and white clothes are popular in the summer. The same basic idea holds true for the planet. Anything that reflects sunlight (ice, white roofs, certain kinds of clouds and air pollution) contributes to cooling; anything that absorbs sunlight (open water, evergreen forests in northern latitudes, asphalt parking lots) contributes to heating.
 In his paper, Crutzen talked specifically about the cooling effect of volcanoes. For years, scientists have known that the sulfate particles that volcanoes spew into the air are remarkably effective at scattering sunlight. If the eruption is large enough, they can have a global impact on temperatures. One of the most recent examples is Mount Pinatubo, a volcano in the Philippines that erupted in 1991, lowering the earth's temperature by a degree or so for several years. A more extreme example of the phenomenon is the so-called nuclear winter - a theory that was much debated in the 1980s, suggesting that a nuclear war could inject enough soot and particles into the atmosphere to block out the sun and send temperatures plummeting.
 Crutzen didn't say how we might go about mimicking volcanoes to offset global warming, except to suggest that there are lots of ways to inject particles into the stratosphere, including spraying them out of high-altitude aircraft, pushing them up a long hose tethered to a stratospheric balloon, or even shooting them up into the sky with artillery. As far as engineering challenges go, it wouldn't be too difficult. And even more important, it would be cheap. In Crutzen's estimation, we could engineer the earth's climate for less than 1 percent of the annual global military budget.
 This all sounded interesting and provocative. It took me a while, however, to grasp just how mind-bending Crutzen's proposal really was. Here was one of the world's top atmospheric scientists suggesting that the climate crisis was so urgent and potentially catastrophic that the only way to save ourselves might be by filling the stratosphere with man-made pollution from artificial volcanoes. Had it really come to this?
 In the media world - at least the part of the media world that takes science seriously - Crutzen's essay raised a ruckus. For one thing, the whole idea of changing the reflectivity of the planet as a way to offset global warming sounded downright wacky, even coming from a serious guy like Crutzen. As for injecting particles into the stratosphere - wasn't the goal to clear the air, not further pollute it? Geoengineering seemed like an idea ripped out of the pages of a sci-fi novel, conjuring up associations with Dr. Evil and crazy Cold War physicists and the hubris of the techno-elite. Perhaps worst of all, Crutzen's argument implied that the whole strategy of relying on an international agreement to cut greenhouse gas emissions was misguided - or at least grossly insufficient.
 This was not a message the world was ready to hear. An Inconvenient Truth, Al Gore's documentary about global warming, had been released the same summer, waking millions of people up to the compelling scientific evidence behind the climate crisis. Progressive politicians around the world were beginning a major push to reduce emissions, trying, at least in public, to give the appearance that they were eager to fulfill their commitment to the Kyoto Protocol, the international agreement to cut greenhouse gas emissions signed in 1997. In Europe, the first market for greenhouse gas emissions trading was just taking off. Financial analysts predicted that the market would someday become the largest in the world, with hundreds of billions of dollars' worth of emissions credits being swapped every year, creating a powerful incentive for power companies to cut pollution and reap the rewards.
 In this context, Crutzen was a turncoat, a man who dared to betray the growing movement to fight global warming just at the moment when it was gaining momentum. “This sounds to me like a miracle fix cooked up by Big Oil to keep the masses fat, dumb and happy,” one blogger commented. “You keep driving and we'll get some smart scientists to air-condition the planet!”
 But Crutzen's logic was not easy to dismiss. If there was one thing I had learned from the four years I'd spent researching and writing about coal, the dirtiest of fossil fuels, it was that the world was not going to stop burning black rocks anytime soon. Coal-fired power plants generate half the electricity in America. In the developing world, the percentage is even higher - India and China both get about 70 percent of their electricity from coal. The Chinese consume almost three times as much coal as we do in the United States - nearly three billion tons a year (although per capita, they consume far less).
 Coal is the engine that is lifting people in the developing world out of poverty, not only giving them the power to light their homes and cook their food but also transforming them, for better or worse, into Western-style consumers. Unfortunately, coal is also the most carbon-intensive of fossil fuels, generating more than a third of the world's CO2 pollution. Everyone wants to be hopeful about the possibilities of the renewable energy revolution, but the truth is, getting off coal in the near future - or, equally unlikely, figuring out a cheap and efficient way to burn coal without releasing CO2 into the atmosphere - is a monumentally difficult challenge. And if we can't get off coal, it doesn't matter if every SUV driver rides a skateboard to work and Al Gore takes over as chairman of ExxonMobil - we won't have a hope in hell of staving off dangerous climate change.
 Equally sobering were the encounters I had with climate scientists during my research. By 2006, the major scientific uncertainties about whether or not the planet was warming - and why it was warming - had long been settled. (I won't bother rehashing the evidence. If you still think global warming is a myth or unrelated to human activity, you're reading the wrong book.) But real questions remained, especially about the rate at which that warming would occur and what the impacts would be. A draft of the 2007 report by the Intergovernmental Panel on Climate Change (IPCC), the United Nations group comprising several hundred top scientists, was already circulating at the time Crutzen's essay appeared. The report predicted that the planet would warm by 3 to 7 degrees Fahrenheit by the end of the century and forecast a future of melting glaciers, rising seas, epic droughts, disease, and famine.
 At the time, the report was touted as the first unequivocal statement from the scientific community about the cause and consequences of global warming. Off the record, however, many scientists were uncomfortable about how conservative the report was. There is good reason, of course, not to overstate scientific consensus or overhype impacts. But many felt it was equally dangerous - and even immoral, given the stakes - to underplay them.
 Today it's clear that those scientists were right to be uncomfortable - the 2007 IPCC report is already woefully out-of-date. Global emissions are rising much faster than the report forecasted, and climate impacts are more severe. Credible studies now indicate that temperatures in the United States could increase by as much as 15 degrees Fahrenheit by the end of the century, with dust bowls in the Southwest and in many other heavily populated regions around the world. And instead of a sea level rise of less than a foot, as the IPCC report suggested, a number of respected climate modelers now believe it could be as high as three feet or more. James Hansen, the director of NASA's Goddard Institute for Space Studies and well-known as the godfather of global warming science, goes even further. He told me during a conversation in 2009 that if we don't cut emissions hard and fast, the seas could rise by as much as nine feet by the end of the century. Goodbye, Bangladesh, London, Miami - and Silicon Valley. If my grandchildren want to visit my hometown, they'll have to put on diving gear. “It would be a different planet,” Hansen said.
 You can see the increasing velocity of the changes most clearly in the Arctic, where winter temperatures in recent years have been as much as 3 degrees Fahrenheit warmer than average. Although that might not sound like a lot, a profound transformation of the region is already under way. According to the National Snow and Ice Data Center, the maximum extent of the summer sea ice cover for 2009 was the third-lowest on record. The six lowest maximum extents since satellite monitoring began in 1979 all occurred between 2004 and 2009. Extend this trend into the future, and the prognosis is not good. Back in 2006, many scientists were predicting that the Arctic would be ice-free in the summer by 2050. Now some scientists believe that it could happen within the next decade. “It's like man is taking the lid off the northern part of the planet,” one polar ice expert commented. The loss of summer sea ice may well be a boon for shipping and oil exploration. But it is also likely to have broad impacts on the earth's climate, especially ocean circulation patterns, which, in turn, could disrupt major climate events such as the Asian and African monsoons. Nearly two billion people depend on those rains to grow their food.
 One of the greatest misapprehensions about the climate crisis is the notion that we can fix all this simply by cutting emissions quickly. We can't. Even if we cut CO2 pollution to zero tomorrow, the amount of CO2 we have already pumped into the atmosphere will ensure that the climate will remain warm for centuries. To understand why, it's important to realize that CO2 is not like the pollutants that create smog, most of which fall out of the air a few days or weeks after they are emitted. CO2 lingers in the atmosphere for a very long time. Every time you drive to the store for a quart of milk, about 50 percent of the CO2 you dump out of the tailpipe remains in the atmosphere for a decade or so before it is absorbed by the earth's carbon cycle. (The oceans are the single largest carbon-
eaters, but plants and trees suck up a lot, too.) It takes a few centuries to absorb the next 30 percent. The final 20 percent lingers in the atmosphere for as long as 100,000 years.
 The implications of this are profound. “The climatic impacts of releasing fossil fuel CO2 to the atmosphere will last longer than Stonehenge,” oceanographer David Archer wrote in 2008. “Longer than time capsules, longer than nuclear waste, far longer than the age of human civilization so far.”
 Besides sucking up carbon, the world's oceans play a big role in the response time of the climate, too. Their waters act like a giant heat sink for the planet. Once the oceans warm up, they will continue to radiate heat for hundreds of years. This thermal inertia, combined with CO2's habit of hanging around in the atmosphere, means we may already be locked into dangerous levels of warming - we just don't know it yet.
 Exactly how much does the climate warm with each additional ton of greenhouse gases we dump into the atmosphere? Scientists can make estimates, based on the heat-trapping properties of a CO2 molecule, but the truth is, no one knows for sure. A key uncertainty in this calculation is the operation of feedback loops - that is, the mechanisms by which a dynamic system like the earth attempts to maintain equilibrium. Positive feedback loops are common. When ice melts in the Arctic, for example, it exposes more open water, which in turn absorbs more heat, which accelerates the warming, which melts more ice - you can see how these feedbacks build on each other. The question is, as positive feedbacks increase and the world heats up, what other changes might they trigger? One fear is that rapidly rising temperatures in the Arctic will cause the permafrost to thaw. Permafrost is loaded with methane, the byproduct of decomposing plants and microbes. If it melts quickly, it could send a sudden pulse of the gas into the atmosphere (methane is a short-lived but potent greenhouse gas, seventy times more powerful than CO2), triggering a massive warming. Another fear is that changing rainfall patterns could cause tropical rain forests to collapse, eliminating a major carbon absorber, or carbon sink, for the planet. Many scientists have looked in vain for negative feedbacks - as yet undiscovered natural systems that will help to counteract the ever-increasing levels of greenhouse gases. So far, they haven't found any - at least not on a scale that matters.
 Then there is the question of what Ken Caldeira, a climate modeler at the Carnegie Institution's Department of Global Ecology at Stanford University, calls the “ringiness” of the climate system. In other words, when you hit the system with a big hammer - and the thirty billion tons of CO2 that humans dump into the atmosphere every year certainly qualify as a big hammer - how much noise does it make? Is this a system that can absorb a big hit, or does the impact amplify throughout the system, perhaps leading to unexpected gyrations? We know that in the past, the climate has jumped from one stable state to another. Consider a climatic event known as the Younger Dryas, which ended about 11,500 years ago, just as the earth was emerging from the last ice age. After several thousand years of warming, temperatures inexplicably plunged 10 to 15 degrees Fahrenheit and stayed that way for a thousand years, before warming up again just as suddenly. During the Younger Dryas, conditions in northern Europe were similar to those in the Arctic today. Icebergs drifted as far south as Portugal. Climate scientists aren't sure what caused this period of dramatic change, but what disturbs them is that if it happened once, it could happen again. Wally Broecker, a pioneering climatologist at Columbia University, has famously compared the earth's climate to a dragon: you poke it, and you're never sure how it is going to react. We could get lucky and be living in a climate system that is more tolerant than we think. But we could also be living in a system that is far more sensitive to being poked than we currently understand. By pushing the system so hard, we are, in effect, playing Russian roulette with the operating system of civilized life.
 A few months after Crutzen's paper was published, I called him at his office at the Max Planck Institute for Chemistry in Mainz, Germany. We briefly discussed his concerns about the essay - a number of respected scientists had cautioned him not to publish it, fearing it would open a Pandora's box of questions about geoengineering - but Crutzen was determined to see it in print. “We live in a fool's climate,” he told me. “We need to prepare for the worst. It might never occur. But if it does, and if we need to cool the earth off in a hurry, what will we do?”

At a dinner party not long ago, I found myself sitting next to the rare books librarian at a nearby college. She expressed concern, as many thoughtful people do, about the consequences of global warming. I mentioned to her that I had recently learned about a group of scientists who believe it might be time to deliberately engineer the earth's climate to counteract global warming. I talked about shooting particles into the sky, cloud machines, and mirrors in space. She looked at me for a moment, not sure whether I was serious or not, then burst out laughing. It was a reasonable response. You don't need a Ph.D. in physics to understand the basic insanity of this undertaking.
 Some climate scientists have a slightly more nuanced view. Back in 2006, I discovered that talking about geoengineering with most scientists was like talking about mining minerals on Mars: an interesting idea, but not one they spent much time pursuing. It's not that they were morally against it. “Scientists love the idea of manipulating systems,” Ken Caldeira told me. “It's how you learn how things work.” The trouble, Caldeira explained, is that few scientists have the funding to simply explore projects that suit their fancies. And back in 2006, there were essentially zero dollars in the mainstream science world to study geoengineering. Why? You would think that Bush-era conservatives and climate skeptics - many of whom controlled the purse strings for science funding circa 2006 - would have loved the idea. And a few did. In late 2001, just months after the Bush administration officially abandoned Kyoto and turned America into not only one of the world's biggest consumers of fossil fuels but also its most unabashed, the U.S. Department of Energy quietly convened a group of energy and climate experts to explore technological responses to rapid climate change. The report from the meeting - which included a pretty good rundown of the pluses and minuses of various geoengineering options - never saw the light of day. It's not hard to see why: to argue for geoengineering with any credibility, it must be combined with a clear call for reductions in greenhouse gas pollution. And until recently, very few conservatives have been willing to make that call.
 Fear of ridicule was another barrier. Although the dream of manipulating the weather is almost as old as civilization itself, the idea of studying ways of deploying technology to manage the earth's climate was seen by some scientists as politically incorrect, dangerous, or just downright silly. Why put your career at risk when there are so many other more fruitful problems to explore? As a consequence, the researchers who explored geoengineering did so as a hobby or as a sideline to their main research projects. It was the scientific equivalent of a porn habit, something you thought about and explored in the privacy of your own lab but did not discuss in polite company, lest you be considered a pervert.
 Indeed, many mainstream scientists were not shy about expressing the view that geoengineering was a crazy idea. “A semi-intelligent visitor from Mars would look down on us and say, You're all totally insane,” said Vaclav Smil, a noted energy expert at the University of Manitoba in Canada. “The fact that we're even talking about it is a sign of desperation,” Michael Oppenheimer, a climate scientist at Princeton, told me. “Foolishness,” said Kevin Trenberth, the head of the Climate Analysis Section at the National Center for Atmospheric Research. John Holdren, who was head of Woods Hole Oceanographic Institution before he took his current job as chief science adviser to President Barack Obama, was equally dismissive. “The geoengineering approaches considered so far appear to be afflicted with some combination of high costs, low leverage, and a high likelihood of serious side effects,” he wrote in 2006. And so it went, on and on. Beltway policy wonks and environmental leaders were even more emphatic in their opposition. David Hawkins, the head of the Climate Center at the Natural Resources Defense Council, called it “the Frankenplanet solution.”
 That was 2006. Today, in the aftermath of the 2009 Copenhagen climate summit, where world leaders talked about the dangers we face from global warming but failed to come up with a legally binding agreement to cut emissions, it's increasingly hard to cling to the idea that we're going to solve this problem with cooperation and good intentions. And as the rhetoric about cutting emissions grows increasingly hollow and the risks of rapid climate change grow ever higher, the public debate is shifting from how to stop global warming to how we can live with it. This means not only reengineering our drinking water infrastructure to prepare for earlier snowmelts and longer droughts and tracking new disease vectors in developing countries, but also beginning to think hard about what we might do if the worst-case scenarios came true and we really were faced with severe droughts that led to famines, or rapidly rising sea levels that threatened major cities and coastal regions. “If we had the climatic equivalent of the subprime economic meltdown, people would demand action,” said Caldeira, who has done many key modeling studies on the impacts of various geoengineering schemes. “The temptation to throw some dust into the stratosphere to cool the planet off in a hurry might be hard to resist.” In this context, geoengineering starts to look less like the fevered dream of mad scientists and more like the fevered dream of panicked politicians. And that is perhaps an even more frightening scenario.
 Nonetheless, the old taboos are fading fast. In 2009, the British Royal Society, one of the most respected scientific organizations in the world, released a major geoengineering study. The U.S. equivalent of the Royal Society, the National Academy of Sciences, is likely to follow suit. Although it's tough to gauge how much support the idea has within President Obama's administration, Steven Chu, the secretary of energy, told me that “geoengineering is certainly worth further research.” He often proselytizes for white roofs on buildings, which, besides saving energy on air conditioning, help (very slightly) to cool the planet by reflecting more sunlight than traditional dark roofs. One of Chu's top appointments at the Department of Energy, Steven Koonin, has long been intrigued by geoengineering and in fact chaired a weeklong scientific conference on the subject in 2008, when he was chief scientist for BP, the London-based petroleum giant. Even the U.S. Congress is sticking its toe in the water. In late 2009, the House Committee on Energy and Technology held the first-ever hearings on geoengineering, signaling that the idea is now safe for public consumption. In addition, money for geoengineering research is starting to flow from private sources, including venture capitalists and technology-loving philanthropists such as Bill Gates. “I'm concerned about the impact of global warming on poor people in the developing world,” Gates told me. “If geoengineering can help reduce that, I think it's worth exploring.”
 In the past year or two, climate skeptics and friends of the fossil fuel industry have also discovered geoengineering. The American Enterprise Institute, which has a long history of working to deny the scientific consensus on climate change and maintains strong ties to the fossil fuel industry (Lee Raymond, former CEO of ExxonMobil, served on its board; ExxonMobil was also a big financial supporter of its climate work), runs one of the few funded policy centers on geoengineering. Other organizations and personalities well-known for derailing serious action on global warming - such as the Cato Institute and Danish statistician Bjørn Lomborg, head of the Copenhagen Consensus Center - have pitched geoengineering as a cheap alternative to cutting emissions. As Alex Steffen, cofounder of Worldchanging, a popular environmental policy and activism website, wrote in April 2009, “Combining dire warnings about climate action's economic costs with exaggerated claims about geoengineering's potential is the new climate denialism.”
 In some ways, geoengineering is a lot like cloning, or genetic engineering, or even nanotechnology. It scrambles old political alliances and carves out new ideological fault lines. It stokes fears about Big Science run amuck, about the limits of human knowledge, about technological progress pushing beyond moral progress. But there are important differences, too. Unlike in these other cutting-edge scientific endeavors, nobody is actually doing any geoengineering yet. At least not in a deliberate way. You could certainly make the argument - and many people do - that civilization itself is a geoengineering project. (Crutzen famously dubbed the past ten thousand years or so “the Anthropocene.”) But the difference is intention. For the past ten thousand years, we could be excused for behaving like locusts, unaware of the larger consequences of our all-consuming appetites. But that excuse is gone now - at least for all of us here in the land of iPhones and air conditioning. Our current greed and recklessness are starting to look a lot like a suicidal impulse.

At any science conference where geoengineering is discussed, it's a good bet that several hours will be devoted to a tortuous debate over what the term “geoengineering” actually means and whether the name should be changed to something softer and less technocratic, such as “climate intervention” or “climate restoration.” For the purposes of this book, I'm going to stick with “geoengineering,” simply because it's the current term of art. As for what exactly the word refers to, the most succinct definition I've encountered comes from that Royal Society paper: “the deliberate large-scale intervention in the Earth's climate system, in order to moderate global warming.”
 And before I go any deeper into this, I must tell you that a lot of the wacky ideas proposed by wannabe geoengineers will not be covered in this book. Two of my favorites: dumping millions of tons of Special K cereal into the ocean to change the reflectivity of the water and ionizing CO2 molecules with lasers so that the earth's magnetic field ejects them from the atmosphere. Crazy ideas, of course, can be fun to read about. But I'm limiting the focus of this book to serious ideas that may be workable in the near term - by which I mean the next fifty years or so. That also eliminates some interesting but wildly impractical proposals, such as launching mirrors into space to deflect sunlight. We may indeed do something like that someday - building what amounts to a permanent louvered sunshade for the earth - but the cost and complexity of such a project are so huge that it is extremely unlikely that it would happen before the end of the century.
 Finally, it's important to make clear from the outset that if we want to geoengineer the planet, there are two fundamentally different ways to do it. One method, sometimes called carbon engineering, focuses on CO2 removal, which includes any technique or technology that pulls CO2 out of the atmosphere, from dumping iron into the ocean in order to stimulate plankton blooms (which in turn absorb CO2) to building scrubbers that remove CO2 from the air. This is by far the least controversial of the two approaches to geoengineering, in part because it's slow acting and essentially mimics the earth's natural carbon cycle. (You could argue that large-scale tree plantations are a form of carbon engineering, too.)
 The second method is to engineer the earth's albedo - to cool the planet by changing its reflectivity, as Crutzen suggested. On the simplest level, you could paint roofs and roads white. Or you could inject particles into the stratosphere, which would be much more effective. Another way would be to build a fleet of machines that would brighten clouds, causing them to scatter more sunlight. In contrast with carbon engineering, albedo engineering - or solar radiation management, as some scientists call it - could be useful in a climate emergency because it would cool the earth off instantly, in the same way that stepping into the shade on a hot day gives you quick relief from the sunlight. Interestingly, you don't have to deflect much sunlight to have a big impact. To offset a doubling of CO2 levels from preindustrial conditions (a common benchmark among climate scientists), you would have to scatter just 2 percent of the light that hits the planet.
 But messing around with the earth's albedo would also be far more ethically fraught than CO2 removal. Deflecting sunlight is not a replacement for reducing CO2 levels in the atmosphere. For one thing, shielding sunlight might lower the temperature, but it would do nothing to solve other problems related to high CO2 levels, such as ocean acidification, which is killing coral reefs and could have a devastating impact on the ocean food chain. Also, because shielding sunlight would change the way heat is distributed around the earth and reduce the differential between daytime and nighttime temperatures, it would likely have broad climatic effects, such as shifting rainfall patterns and intensity. (Of course, unchecked global warming is also likely to have a big effect on rainfall.) Perhaps the most dangerous aspect of this kind of geoengineering is that throwing a few million tons of dust into the stratosphere would be relatively quick, cheap, and easy to do, putting it well within reach of petty dictators and tyrants.
 If we ever got serious about geoengineering the planet, we would be likely to deploy a variety of both carbon engineering and sunlight-blocking technologies. The real question is not how we would do it. It's should we do it. I discovered that the case against deliberately manipulating the earth's climate really boils down to three arguments.
 First, we are messing with a system we don't understand. The earth's climate is immeasurably complex - we may have fancy climate models and a passing understanding of atmospheric chemistry, but our ignorance remains vast. For example, scientists understand the basic physics of cloud formation, but beyond that, much of what happens in the clouds remains a mystery. If we don't even understand clouds, how can we hope to understand the complex interactions between earth and sky that shape the highly dynamic system we call climate? As David Battisti, a climate modeler at the University of Washington, put it, “Think of the climate like your car - the faster you go, the more likely you are to feel the wheels start to wobble. And the more you push it, the faster you go, the more likely it is to spin out of control.” If the twentieth century taught us one thing, it's that technological innovations always have unintended consequences. Bomb builders at Los Alamos thought they were working on a device to bring world peace, but they also unleashed nuclear proliferation and fear of annihilation. The automobile brought personal freedom, but it also gave us strip malls, suburbs, and global warming. Dams control floods, create reservoirs, and generate clean power. They also destroy rivers, threaten migratory fish, and encourage mindless consumption of water.
 What kind of havoc might geoengineering bring? You name it. Environmentally, the biggest concerns are shifting precipitation patterns and unexpected droughts, which could be devastating for many food-producing regions of the world. Foreign policy experts fear conflicts over nations “stealing” one another's rain. Military leaders worry about climate warfare. Human rights activists can already see a world in which the rich will use their technological superiority to screw the poor. If it comes down to a choice between rain in Africa or rain in Iowa, which region do you think is going to win?
 Then there are the psychological consequences. What happens when the color of the sky on a particular day is the result not of Mother Nature's mood but of the mood of geoengineers who are spreading dust in the stratosphere? (Because of their light-scattering effects, high-altitude particles are likely to cause paler skies but more brightly colored sunsets.) What happens to our romance with Nature - sentimental as it may be at times - when we become hyperconscious that we are all living in a terrarium?
 The second argument against geoengineering is that even talking about it distracts us from the urgent job of cutting greenhouse gas pollution. “It's like the way Baptists view sex education in school,” Steve Rayner, professor of science and civilization at the University of Oxford, said at a recent geoengineering conference. “The worry is, if you start talking about it, you're more likely to do it.” According to this view, if people believe there is a quick technological fix out there for global warming, they will ask why we should bother going through all the pain and struggle of reinventing the world's energy systems. After all, who wants to pay higher electric bills, move to a smaller house, or give up their third TV if we can just throw some dust in the air and cool off the planet? This is a version of the classic “moral hazard” argument that economists use frequently to underscore why flood insurance encourages people to build homes in flood-prone locations, or why bank bailouts discourage investment firms from instituting real reforms. If someone else is going to cover the loss, it greatly lessens the urgency of taking responsibility for one's own actions. And what if the fix doesn't work as planned? It's one thing to smoke in bed, confident that if anything happens, you can dial 911 and the fire department will be there in two minutes. But what if when the firefighters arrive, the pumps and fire hoses don't work?
 In fact, betting on geoengineering as a substitute for cutting emissions is a really bad idea for a number of reasons. For one, carbon engineering alone is unlikely to soak up more than a modest percentage of the CO2 we dump into the atmosphere burning fossil fuels. And when it comes to deflecting sunlight, there is what Ray Pierrehumbert, a climate researcher at the University of Chicago, calls “the Sword of Damocles problem.” If we begin throwing dust into the stratosphere to scatter sunlight, we have to keep doing it. Unlike CO2, particles that are injected into the stratosphere will fall out of the sky after a year or so. If we don't inject more, the sky will clear, which will trigger a sudden warming - just like stepping out of the shadows into the sunlight. So it comes down to a question of intent. If we inject particles into the sky as a way to keep Greenland from melting too quickly, or to buy us a few extra decades to reduce emissions, it might make sense. If it's simply a lazy way of continuing with business as usual, however, we're likely to make our problems much worse. “If we keep emitting greenhouse gases with the intent of offsetting the global warming with ever-increasing loadings of particles in the stratosphere, we will be heading to a planet with extremely high greenhouse gases and a thick stratospheric haze that we would need to maintain more or less indefinitely,” Caldeira told me. “This seems to be a dystopian world out of a science fiction story.”
 The third and, in my view, strongest argument against geoengineering is that it's evidence of exactly the same kind of industrial thinking that cooked the planet in the first place. The fundamental challenge of global warming goes far beyond just cutting emissions. It involves changing everything about our lives, from where we live and how we work to our definitions of progress and prosperity. And this is the real problem with geoengineering. Instead of reducing our voracious appetites for material goods and inspiring us to lead simpler lives, it compels us to chase after a technological fix - a high-tech Band-Aid that will solve all our problems.
 The simple and obvious fact is that Western civilization as we know it is unsustainable. We are running out of cheap oil; we are overfishing the oceans; we are depleting our soils; we are running short of drinking water. Geoengineering is not going to fix all that. The only way to build a sustainable planet is to fundamentally reinvent our lives, either accelerating forward into a radically different future or falling back into a more primitive past. The virtue of riding out the climate crisis uncushioned by geoengineering, some would argue, is that it could provide the shock we need to sober up. That's an ugly way to think about it, but certainly there are many people who hold this view, whether they consciously admit it or not.
 These are all compelling arguments for why geoengineering is a dangerous idea. But they aren't compelling enough for me to dismiss it entirely. Yes, progress is always a devil's bargain, but I am suspicious of full-frontal attacks on the evils of technological fixes. Catalytic converters, which remove pollutants from the tailpipes of cars, have saved tens of millions of lives. Antibiotics kill infections. Mosquito nets prevent malaria. Prosthetic knees, hips, and legs give people mobility. Milk from genetically engineered goats is used to manufacture lifesaving drugs. Computers process information - including enormously complex climate models, which allow us to speculate intelligently about just how much trouble our technological society has gotten itself into.
 Similarly, geoengineering could turn out to be an important tool for risk reduction. If the climate crisis turns out to be even worse than we can now imagine, reflecting sunlight away from the planet would be one of the few options we would have to cool things off in a hurry. This doesn't mean we should start building cloud-brightening machines tomorrow, but it certainly does suggest that it might be prudent at least to begin to research these options in earnest - if only so that we can discover what doesn't work now before we urgently need a fix.
 Still, the idea of pursuing geoengineering further - of taking a few years to write a book about it - did not really occur to me until I called David Keith, the head of the energy and environment program at the University of Calgary in Alberta, Canada. Keith, I knew, was a highly respected scientist studying ways to store CO2 from coal-fired power plants underground in deep saline aquifers. What I didn't know was that he had also been thinking and writing about geoengineering for more than a decade. A quick telephone conversation convinced me that he knew as much about the moral, political, and engineering complexities of this idea as anyone I had encountered. More important, he was, at that very moment, building a machine to scrub CO2 out of the atmosphere. “In fact, the machine will be up and running in a few days,” he said. “You're welcome to come and see it if you want.”
 As a child of Silicon Valley, I've always had a soft spot for big thinkers and far-out hardware. And if you drew up a short list of inventions that could change the world, a machine that removes CO2 would certainly be high on the list. It does not take much imagination to see that a machine like that - or, more accurately, a few thousand machines like that - could someday function as part of a climate control system. How warm would you like your planet, sir? Just set the thermostat at the CO2 level you like, fine-tune the sunlight with some dust in the stratosphere or some machine-brightened clouds, and you have your Goldilocks climate - not too hot, not too cold, just right.
 Of course, it wouldn't be quite that simple, but that was the promise. Was Keith's machine another step toward the cliff of extinction or a parachute for the human race? A week later, I jumped on a plane to Calgary to find out.